In the b-galactosidase assay, it is important to measure the density of e. coli cells because select one:
a. the lac operon is induced only in a small percentage of cells in a culture.
b. the transformation efficiency of e. coli cells is low.
c. e. coli densities vary among cultures and with time, which will affect estimates of b-galactosidase production rates.
d. onpg production by e. coli is a function of cell density.
e. we are measuring per cell cleavage of x-gal in relation to culture conditions and time.

Option C, E. coli densities vary among cultures and with time, which will affect estimates of b-galactosidase production rates.

Explanation:

The activity of β-galactosidase  depends on the optical density or the intensity of the color.

In regards to color, yellow color intensity signifies the qualitative parameter of the β-galactosidase activity while the readings obtained through colorimeter is quantitative parameter of the β-galactosidase activity. In the presence of lactose , the gene responsible for producing β-galactosidase in E. coli is either induced or switched off.

Hence , option D  is correct

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the law of original horizontality explains:   why rocks appear in layers. correct answer: b the law suggests that all rock layers are originally laid down (deposited) horizontally and can later be deformed.   the principle is used by geologists to make assumptions about the relative ages of sedimentary rocks.